When a tooth is damaged due to decaying and when a cavity caused by the damage is still relatively small, the tooth is, in many cases, directly restored by using a composite resin from the standpoint of aesthetic appearance, simplicity and quickness of operation. When the cavity caused by the damage is relatively large, on the other hand, the cavity is restored by using a prosthetic material prepared by using a metal, ceramics or a cured resin material.
The dental restorative such as a composite resin or a prosthetic material has no adhesiveness to the tooth. Therefore, the restorative is adhered to the tooth by using an adhesive which comprises a polymerizable composition. The polymerizable composition used as the adhesive, usually, contains a methacrylate-type monomer as the polymerizable monomer which is a chief component but its adhesive force to the tooth is not sufficient. For example, when the polymerizable composition is used for adhering the composite resin to the tooth, the adhesive strength that is obtained is not, in many cases, large enough to overcome the internal stress (tensile stress occurring in the interface between the tooth and the composite resin) that generates when the composite resin cures. In many cases, further, the adhesive strength is not large enough to withstand the force produced by occlusion.
In order to improve the adhesive strength of the adhesive, therefore, at the time of using the adhesive, the tooth surface is pre-treated, such as:                (a) a hard tooth (enamel comprising chiefly hydroxyapatite) is etched, and        (b) an adhesiveness-improving component called primer is penetrated into the tooth.        
Here, an aqueous solution of acid for demineralizing the tooth surface is usually used as a pre-treating agent for etching. Namely, an aqueous solution of phosphoric acid, citric acid or maleic acid is used.
After the etching (demineralizing), further, a dentin is exposed on the surface of the tooth, the dentin comprising an enamel or spongy collagen fiber that is coarsely granulated due to demineralizing with the acid aqueous solution. To maintain a sufficiently large adhesive strength, however, the adhesive component must penetrate through the tooth surface and curing must be performed in this condition. As the primer for the penetration treatment, therefore, there has been used a polymerizable monomer composition which contains an organic solvent or a hydrophilic monomer having good affinity to the tooth, such as hydroxyethyl methacrylate (HEMA). Though the primer by itself does not usually contain any polymerization initiator, the polymerizable monomer in the primer is polymerized and cured by the action of radicals formed by the adhesive at the time when the adhesive for the composite resin applied thereon undergoes the photo-curing reaction.
While the pre-treatment is conducted as described above, there have been developed various dental adhesive compositions containing a polymerizable monomer having adhesiveness to the tooth in order to attain higher adhesive strength and to reduce complexity of pre-treating operation.
For example, patent documents 1 and 2 propose dental adhesive compositions containing an acidic group-containing polymerizable monomer as at least part of the polymerizable monomer component. These adhesive compositions exhibit higher adhesive strengths since the acidic group-containing polymerizable monomer having an acidic group such as phosphoric acid group or carboxylic acid group in the molecules thereof has a high affinity to the tooth (hydroxyapatite or collagen).
Further, patent documents 3 to 6 propose adhesive compositions in which an acidic group-containing polymerizable monomer is present together with water. These adhesive compositions exhibit both the demineralizing function due to the acid aqueous solution based on the action of the acidic group and the primer penetration accelerating function, and eliminate the need of separately applying a pre-treating agent. Namely, these adhesive compositions can be used upon being applied only once, and can be advantageously used as adhesives (one-step type adhesives) featuring excellent operability. Further, by also utilizing their etching property and penetrating property, the adhesive compositions containing the acidic group-containing polymerizable monomer can be used as self-etching primers having the above two functions (e.g., see patent document 8 and patent document 9).
A patent document 10 reports that the adhesive strength can be further improved if a polymerizable monomer having a phosphoric acid group as an acidic group is used in the form of a calcium salt.
Among the above various prior arts, patent documents 4 to 7 propose adhesives and self-etching primers containing an acidic group-containing polymerizable monomer and water, which are blended with an earth metal ion-eluting fillers. Here, the earth metal ion-eluting filler is a filler such as a fluoroaluminosilicate glass, that elutes out metal ions in an acidic solution. The metal ions eluted out therefrom are the ions of alkaline earth metals and aluminum. The adhesive compositions containing the filler that elutes out earth metal ions are supposed to further improve the adhesive strength presumably due to that the polymerizable monomer component containing the acidic group-containing polymerizable monomer is cured by polymerization and, besides, multivalent metal ions eluted out from the earth metal ion-eluting filler undergo the ionic crosslinking with the acidic groups of the acidic group-containing polymerizable monomer causing an increase in the strength of the obtained cured body.    Patent document 1: JP-A-52-113089    Patent document 2: JP-A-58-21687    Patent document 3: JP-A-2004-352698    Patent document 4: JP-A-9-263604    Patent document 5: JP-A-10-236912    Patent document 6: JP-A-2001-72523    Patent document 7: JP-A-2000-86421    Patent document 8: JP-A-7-82115    Patent document 9: JP-A-2000-159621    Patent document 10: JP-A-53-113843